The Hadley circulation is
changing in response to a warming in the tropical Indian Ocean and Pacific
Ocean during the past 50 years. The oceanic warming has two manifestations: the
increased amplitude of warm sea surface temperature (SST) anomalies during El
Niño in the central and eastern Pacific Ocean; and the slow monotonic increase
of the SST in the tropical Indian and western Pacific Oceans. Evidence for the
changes in the Hadley Circulation comes from ensemble simulations of an
atmospheric general circulation model (AGCM) forced with the observed evolution
of SST for the period from 1950 to 1999, station observations of precipitation,
and the NCEP reanalysis data.

We measure the Hadley Circulation
by the zonal average of the 200mb meridional velocity minus the zonal average
of the 850mb meridional velocity, which contrasts the opposite flows of the
upper and lower tropospheric mass translation. Measured as such, the
climatological Hadley Circulation has a strong annual cycle typified by upper
level mass transport into the Northern Hemisphere during boreal winter and into
the Southern Hemisphere during boreal summer.The changes in the Hadley Circulation appear to be also strongly
seasonal dependent. As shown in Figure 1
, the simulated Hadley Circulation is
intensified during the northern winter (e.g. December-January-February, or DJF)
since 1950 while no long-term trend exists for the northern summer (e.g.
June-July-August, or JJA). The DJF intensification of the Hadley Circulation
can be further partitioned into El Niño-related and non-El Nino-related
portions. Figure 2 shows the linear regression between the Hadley Circulation
and the spatial average of SST anomalies for the region (10ºS-10ºN, 160ºW-80ºW)
(Fig. 2, left column) and the residual from the linear regression (i.e. total
value minus regressed value) (Fig. 2, right column). About half of the DJF
intensification of the Hadley Circulation can be explained by the linear
response to the increased amplitude of El Niño in the central and eastern
tropical Pacific Ocean. The remaining part is attributed to the warming in the
tropical Indian and western Pacific Oceans, which is more interdecadal trend
rather than increase in the magnitude ofinterannual variability.

The seasonal dependence of the
change in the Hadley Circulation since 1950 reflects different impacts of the
SST warming on the oceanic-dominated monsoons during boreal winter versus the
continental-dominated monsoons during boreal summer. During winter, a warming
of the oceans throughout the deep tropics yields an increase in zonal averaged
rainfall (not shown), and hence an intensification of the zonally symmetric
meridional over turning. The summertime response of the monsoon is more
regional, and lacks a zonally symmetric component.

The regional patterns of the
simulated interdecadal changes in winter and summer 200mb divergent mass
circulations are shown in Figure 3.
The oceanic warming
enhances the ocean-land thermal contrast during the
northern winter, therefore
intensifies winter monsoon circulation over the Indian Ocean and Asian Monsoon
region. The intensified winter monsoon circulation, together with the enhanced
convections over the southeastern tropical Pacific Ocean makes the Hadley
Circulation intensified during the northern winter (Fig. 3, top panel). During
the northern summer, the oceanic warming forces intensification of convection
over the tropical Indian and western Pacific Ocean, which in turn causes the
intensification of descending motions over North Africa and helps the
development of persistent summertime drought over that region during the past
several decades. The intensification of convection over the tropical Indian and
western Pacific Ocean is also accompanied by stronger subsidence over the
northern tropical American monsoon area during the northern summer (Fig. 3,
bottom panel). In addition, stronger convection over the ocean weakens the
monsoon circulation in the tropical monsoon regions and is associated with a
general trend of reduced precipitation over land in the tropical region of
Asian Monsoon and the marine land of Philippine and Indonesia (not shown). In
the extratropics, the enhanced Hadley Circulation is linked to the summer
monsoons over Northeast Asia and North America by the intensification of
subtropical high and the strengthening and southward shift of the westerly jet
over the Northern Hemisphere.